In association with the OB2020 seismic survey, over 8,200 line kilometre of gravity and magnetic data were acquired. These data were subsequently merged with existing satellite data to produce merged grids at 1000m grid cell size. Several enhancement processing techniques were applied to these magnetic and gravity data to better highlight buried features within the Otway Basin. The merged input data from the survey and the enhanced products in this release provide valuable information on the geometry and spatial extent of igneous rocks in the deep-water basin. The distribution of these rocks is critical to the understanding of the petroleum systems and therefore the hydrocarbon prospectivity of the area. This data package contains: 1) A metadata statement document 2) Shapefiles of the magnetic and gravity line data from the OBSP survey 3) ASCII xyz grids of the OBSP and merged grids with public domain data 4) Georeferenced (GeoTIFF) images of the survey and merged grids 5) Gravity and Magnetic data processing reports from the OBSP survey

<div>The Sherbrook Supersequence (Campanian–Maastrichtian) is the youngest of four Cretaceous supersequences in the Otway Basin and was deposited during a phase of crustal extension. Supersequence thickness is typically less than 1000 ms TWT across the inboard platform. Beyond the platform edge up to 2 800 ms TWT of Sherbrook sediments were deposited in the deep-water Morum and Nelson sub-basins. Analysis of wireline-logs and cores from wells yielded fluvial, deltaic, coastal shelf gross depositional environments (GDEs). As the number of regionally mappable seismic facies is much less than the number of well-based GDEs, the integration of well-based environmental interpretations with seismic facies resulted in three main regional GDE (RGDE); Fluvial Plain, Coastal/Delta Plain, and Shelf. The Fluvial Plain and Coastal/Deltaic RGDEs are almost entirely restricted to the inboard platform areas of the basin. The mud-prone Shelf RGDE is widespread across the deep-water part of the basin where it forms the depocentres of the Morum and Nelson sub-basins. The Shelf RGDE is well imaged on the Otway 2020 2D seismic data that was acquired over the deep-water Otway Basin. In the Morum Sub-basin, the Shelf RGDE is strongly influenced by growth on extensional faults. In contrast, the Shelf RGDE in the Nelson Sub-bsin is a relatively unstructured progradational complex. The presence of mass-transport and incision complexes are consistent with active tectonism during Sherbrook deposition. Reservoir rocks in the deep-water basin are best developed in the Coastal/Deltaic RGDE where it encroaches into the Morum Sub-basin, and where the Austral 3 petroleum system was potentially active within the Sherbrook Supersequence.&nbsp;</div> This presentation was given at the 2023 Australasian Exploration Geoscience Conference (AEGC) 13-18 March, Brisbane (https://2023.aegc.com.au/)

<div>Gas production from the Inner Otway Basin commenced in the early 2000s but the deep-water part of this basin remains an exploration frontier. Historically, the understanding of plays in this region were largely model driven and therefore the ground-truthing of depositional environments (DE) and gross depositional environments (GDE) are critical. This aspect has been investigated for the Sherbrook Supersequence (SS) by the integration of legacy wireline and core data, with regional 2D seismic facies mapping of new and reprocessed data from Geoscience Australia’s 2020 Otway Basin seismic program. Core observations were matched to wireline logs and seismic facies with resulting well based DE interpretations calibrated to seismic resolution Regional GDE intervals. Integration of well and seismic observations lead to the compilation of a basin-wide Regional GDE map for the Sherbrook SS. This GDE map indicates the distribution of Sherbrook SS play elements such as source rock, seal and reservoir, especially across the Deep Water Otway Basin where well data is sparse.</div> Published in The APPEA Journal 2023. <b>Citation:</b> Cubitt Chris, Abbott Steve, Bernardel George, Gunning Merrie-Ellen, Nguyen Duy, Nicholson Chris, Stoate Alan (2023) Cretaceous depositional environment interpretation of offshore Otway Basin cores and wireline logs; application to the generation of basin-scale gross depositional environment maps. <i>The APPEA Journal</i><b> 63</b>, S215-S220. https://doi.org/10.1071/AJ22090

<div>Exploring for the Future (EFTF) is a program dedicated to exploring Australia’s resource potential and boosting investment. This program is designed to produce pre-competitive information to assist with the evaluation of the hydrocarbon resource potential of onshore basins and attract exploration investment to Australia. This record presents geochemical analyses of natural gases sampled from Nangwarry 1, located in the onshore Otway Basin, undertaken in partnership with the Department for Energy and Mining – Energy Resources, Government of South Australia, as part of the EFTF program Natural Hydrogen module. The Nangwarry Joint Venture drilled Nangwarry 1 to investigate the potential for the development of food grade, carbon dioxide production from this well. The results of the molecular and stable carbon and hydrogen isotopic analyses undertaken by Geoscience Australia are released in this report. The molecular data show that the gas composition in this well has an average of 96 mol% CO2 with an isotopic signature indicative of a magmatic origin, being comparable with previously produced gases from onshore Otway Basin wells (e.g. Boggy Creek 1, Caroline 1) for use by the food industry. The carbon and hydrogen isotopic composition of the C1–C5 hydrocarbon gases from Nangwarry 1 are suggestive of a source from within the Crayfish Supersequence.</div>

The Otway Basin is a broadly northwest-southeast trending basin and forms part of a rift system that developed along Australia’s southern margin. It represents an established hydrocarbon province with mostly onshore and shallow-water offshore discoveries. However, the outboard deep-water Otway Basin, with water depths up to 6300 m, is comparatively underexplored and can be considered a frontier area. Following the completion of a basin-wide seismic depth-imaging program (Part 1; Lee et al 2021) and insights from the revised seismic interpretation (Part 2; Karvelas et al. 2021), we have developed a comprehensive petroleum system modelling (PSM) study by integrating these data and findings (Part 3). Together the studies have resulted in an improved understanding of the hydrocarbon prospectivity of the deep-water areas of the basin. Given the sparsity of data outboard, almost all legacy petroleum system modelling studies have been focused either on the onshore or shallow-water areas of the basin and primarily on their thick Lower Cretaceous depocentres. The limitations of legacy seismic datasets resulted in a high degree of uncertainty in the derivative interpretations used as input into PSM studies. In addition, the paucity and poor quality of data in the deep-water area reduced confidence in the understanding of the basin evolution and spatial distribution of depositional environments through time. The newly acquired 2D seismic survey and reprocessed legacy data, with calibration via several wells across the basin, has improved confidence in our understanding of the tectonostratigraphic evolution of the basin (Part 2; Karvelas et al. 2021). The study presented herein integrates products from the work in Part 2 into a petroleum system model with the primary objective being to better understand the petroleum systems across the deep-water Otway Basin.

<div>This data package contains interpretations of airborne electromagnetic (AEM) conductivity sections in the Exploring for the Future (EFTF) program’s Eastern Resources Corridor (ERC) study area, in south eastern Australia. Conductivity sections from 3 AEM surveys were interpreted to provide a continuous interpretation across the study area – the EFTF AusAEM ERC (Ley-Cooper, 2021), the Frome Embayment TEMPEST (Costelloe et al., 2012) and the MinEx CRC Mundi (Brodie, 2021) AEM surveys. Selected lines from the Frome Embayment TEMPEST and MinEx CRC Mundi surveys were chosen for interpretation to align with the 20&nbsp;km line-spaced EFTF AusAEM ERC survey (Figure 1).</div><div>The aim of this study was to interpret the AEM conductivity sections to develop a regional understanding of the near-surface stratigraphy and structural architecture. To ensure that the interpretations took into account the local geological features, the AEM conductivity sections were integrated and interpreted with other geological and geophysical datasets, such as boreholes, potential fields, surface and basement geology maps, and seismic interpretations. This approach provides a near-surface fundamental regional geological framework to support more detailed investigations. </div><div>This study interpreted between the ground surface and 500&nbsp;m depth along almost 30,000 line kilometres of nominally 20&nbsp;km line-spaced AEM conductivity sections, across an area of approximately 550,000&nbsp;km2. These interpretations delineate the geo-electrical features that correspond to major chronostratigraphic boundaries, and capture detailed stratigraphic information associated with these boundaries. These interpretations produced approximately 170,000 depth estimate points or approximately 9,100 3D line segments, each attributed with high-quality geometric, stratigraphic, and ancillary data. The depth estimate points are formatted for compliance with Geoscience Australia’s (GA) Estimates of Geological and Geophysical Surfaces (EGGS) database, the national repository for standardised depth estimate points. </div><div>Results from these interpretations provided support to stratigraphic drillhole targeting, as part of the Delamerian Margins NSW National Drilling Initiative campaign, a collaboration between GA’s EFTF program, the MinEx CRC National Drilling Initiative and the Geological Survey of New South Wales. The interpretations have applications in a wide range of disciplines, such as mineral, energy and groundwater resource exploration, environmental management, subsurface mapping, tectonic evolution studies, and cover thickness, prospectivity, and economic modelling. It is anticipated that these interpretations will benefit government, industry and academia with interest in the geology of the ERC region.</div>

Reports of bitumen stranding on the ocean beaches of southern Australia date back to the early days of European settlement. Previous investigations have shown that this ‘coastal bitumen’ comprises three categories of stranded petroleum: waxy bitumen, asphaltite and oil slicks. All three varieties are physically and chemically distinct from each other, and bear no geochemical resemblance to any indigenous Australian crude oil. This study focuses on the most common variety, waxy bitumen, which accounted for 90% of the strandings on six South Australian beaches repeatedly surveyed during 1991–1992. Geochemical analysis of 96 individual specimens collected from these survey sites and other beaches in South Australia and western Victoria has shown them to be variously weathered high-wax crude oils of paraffinic to aromatic-intermediate bulk composition. Elemental, isotopic and biomarker differences allow their assignment to at least five oil families with inferred source facies that range from deep freshwater lacustrine through paludal and deltaic to euxinic marine, possibly deposited within different sedimentary basins. Family 1, 2 and 3 waxy bitumens all contain biomarkers derived from the freshwater alga Botryococcus sp. and tropical angiosperms (notably dipterocarps). Similar biomarker assemblages are unknown in Australian sedimentary basins but are common in Cenozoic crude oils and source rocks throughout western Indonesia. Family 4 waxy bitumens lack these biomarkers, but do contain dinosterane and 24-n-propylcholestane, indicative of a marine source affinity, while the carbon isotopic signatures and high pristane/phytane (Pr/Ph) ratios of Family 5 waxy bitumens are consistent with their origin from coal-rich source rocks deposited in fluvial to deltaic sedimentary successions. The majority of these waxy bitumens represent an oceanic influx of non-indigenous, Southeast Asian crude oils carried into the waters of southern Australia by the Leeuwin Current. Although they are likely to originate from natural seepage within the Indonesian Archipelago, it is unknown whether the parent oils emanate from submarine seeps or from inland seepages which are then carried to the sea by rivers. The common practice of tanker cleaning operations in the Java and Banda seas may augment the supply of natural bitumen to the beaches of Australia.

Exploring for the Future (EFTF) is a multiyear (2016–2024) initiative of the Australian Government, conducted by Geoscience Australia. This program aims to improve Australia’s desirability for industry investment in resource exploration of frontier regions across Australia. This paper will focus on the science impacts from the EFTF program in northern Australia derived from the acquisition and interpretation of seismic surveys, the drilling of the NDI Carrara 1 and also complementary scientific analysis and interpretation to determine the resource potential of the region. This work was undertaken in collaboration with the Northern Territory Geological Survey, the Queensland Geological Survey, AuScope and the MinEx CRC. These new data link the highly prospective resource rich areas of the McArthur Basin and Mt Isa Province via a continuous seismic traverse across central northern Australia. The Exploring for the Future program aims to further de-risk exploration within greenfield regions and position northern Australia for future exploration investment. [Carr] The Sherbrook Supersequence is the youngest of four Cretaceous supersequences in the Otway Basin and was deposited during a phase of crustal extension. This presentation shows how a basin-scale gross depositional environment (GDE) map for the Sherbrook SS was constructed, the significance of the map for the Austral 3 petroleum system, and why GDE mapping is important for pre-competitive basin studies at Geoscience Australia. [Abbott]

Geoscience Australia has undertaken a regional seismic mapping study of the offshore Otway Basin extending across the explored inner basin to the frontier deep-water region. Seismic interpretation covers over 18,000 line-km of new and reprocessed data acquired in the 2020 Otway Basin seismic program, over 40,000 line-km of legacy 2D seismic data and GA’s new 2023 Otway 3D post-stack Mega Merge seismic dataset. This work provides a new perspective on regional structural architecture and basin evolution and has important implications for hydrocarbon prospectivity of this region. This seminar was two short talks centring on the Otway Basin. <u>Post-stack 3D merging to fast-track regional interpretation - offshore Otway Basin case study, presented by Merrie-Ellen Gunning</u> This case study was to produce a regularised and seamless 3D dataset of the highest possible quality, for the offshore Otway Basin, within two-months. The input migrated volumes varied by data extent, migration methodology, angle range and grid orientation. Fourteen input volumes totalling 8,092 km2 were post-stack merged and processed to produce a continuous and consistent volume, enabling more efficient and effective interpretation of the region. The surveys were regularised onto a common grid, optimised for structural trend, prior to survey matching. A mis-tie analysis algorithm, applied over a time window optimised for interpretation of key events, was used to derive corrections for timing, phase and amplitude, using a reference. This was followed by time-variant spectral and amplitude matching to improve continuity between volumes. Additional enhancements including noise removal and lateral amplitude scaling were also applied. The final merged volume offers significant uplift over the inputs, providing better imaging of structure and events and dramatically improving the efficiency and quality of interpretation. This enables rapid reconnaissance of the area by explorers. <u>Structural architecture of the offshore Otway Basin presented by Chris Nicholson</u> We present new basin-scale isochore maps that show the distribution of the Cretaceous depocentres. Maps for the Lower Cretaceous Crayfish and Eumeralla supersequences, together with those recently published for the Upper Cretaceous Shipwreck and Sherbrook Supersequences, completes the set of isochore maps for the main tectonostratigraphic basin intervals. Mapping of basement involved faults has revealed structural fabrics that have influenced depocentre development. The tectonostratigraphic development of depocentres and maps of deep crustal units delineate crustal thinning trends related to late Cretaceous extension phases. This work highlights the need to review and update structural elements. For example, the boundary between the Otway and Sorell basins is now geologically constrained. The refinements to the tectonostratigraphic evolution of the Otway Basin presented here have important implications for the distribution and potential maturity of petroleum systems, especially with regard to heat flow associated with crustal extension.

Over 8,200 line kilometres of gravity and magnetic data, acquired during the 2020 Otway Basin Seismic Program (OBSP), were combined with public domain survey and satellite data to produce seamless maps of the NW-SE trending deep-water Otway Basin. These data provide valuable information on the geometry and spatial extent of igneous rocks in the deep-water basin. While the top of basement can effectively be imaged from seismic reflection datasets onshore in the Otway Basin, it remains problematic in parts of the deep-water offshore region due to variable seismic data quality. Modelling of the magnetic line data provides an estimate of the depth to the top of basement, an important interface for understanding hydrocarbon prospectivity because it plays a key role in characterising the tectonic evolution of the basin, and thus the thermal maturation history of hydrocarbons. Magnetic modelling was performed using a profile-based curve matching technique producing a depth estimate to the top of the magnetic body that is assumed to be the top of the basement. However, this assumption is flawed where there are volcanic or igneous intra-sedimentary rocks in the basin, as is the case for the Otway Basin where the interpretation of seismic reflection data shows highly reflective events corresponding to igneous features. In most parts of the basin, the modelling results show two layers: a shallow layer (depths < 1000m) corresponding to near surface volcanics, and a deeper layer (depths > 1000m) attributed to the top of the magnetic basement. Magnetic basement shows some similarities with basement picked on seismic reflection data, though in some areas the magnetic basement is shallower. The results also show that the depth to basement is not well resolved in areas with abundant intra-sedimentary igneous rocks. Further investigation is needed in such areas. Presented at the 2024 Australian Society of Exploration Geophysicists (ASEG) Discover Symposium